1. Field of the Invention
The present invention relates to an endless intermediate transfer belt to which multiple toner images formed of a single color on multiple image forming members are overlappingly transferred. The toner images form a multi-colored image which is transferred to a transfer material. An image forming apparatus including the intermediate transfer belt, an image forming method using the intermediate transfer belt and a method of manufacturing the intermediate transfer belt are also described.
2. Description of the Background
To obtain multi-colored images by using an image forming apparatus such as a photocopier and a printer, the following processes are typically used:    (1) Forming images of a single color on image bearing members;    (2) Sequentially overlappingly transferring each color image to an intermediate transfer belt serving as intermediate transfer medium; and    (3) Electrostatically transferring the image on the intermediate transfer belt to a transfer material simultaneously.
The image forming apparatus mentioned above has been improved with regard to the speed of photocopying and the image quality and is now expected to have the same image quality as that of printing machines with regard to multiple-color overlapped images.
Published unexamined Japanese Patent Application No. (hereinafter referred to as JOP) 2002-244391 discloses an image forming apparatus which can prevent registration shift during intermediate transfer by controlling the rotation speed of multiple image bearing members and the surface speed of an image transfer belt to form images with good quality without color shift.
In addition, JOP11-109761 discloses an intermediate transfer belt for use informing electrophotographic images which is flow-cast to form an endless belt by centrifugal-molding single-layer structured materials formed of thermosetting resins having a high temperature resistivity such as polyimides, polyamideimides and electroconductive carbon which is dispersed in the resin.
Further, JOP 2003-145561 discloses a technology by which aromatic polyimides having an inflexible structure are highly drawn to orient molecules.
However, the inventions mentioned above have the following drawbacks.
It is practically impossible to eliminate the difference between the rotation speed of multiple image bearing members and the surface speed of an intermediate transfer belt, even after these speeds are controlled. Registration shifts of color images on an intermediate transfer belt thus tend to occur, especially when the belt has a low elasticity modulus. As a result, good image quality is not obtained.
In recent years, more and more photocopiers and printers include a four-tandem engine system (significantly increasing speed), in which a color image is formed in one pass, instead of a single engine system (printing speed is slow), in which a transfer drum must rotate four times to form one color image.
However, since a four color toner image is formed on an intermediate transfer belt in one pass, there is a color shift in the image finally obtained because of slackening between the right hand side portion and the left hand side portion of the intermediate transfer belt, and flexure of the belt between each transfer drum. These problems are caused by transient decrease in the speed of the intermediate transfer belt due to the contact between the intermediate transfer belt and transfer drums for each color.
In addition, in JOP 2003-145561, the technology disclosed therein is not applied to an intermediate transfer belt using a polyimide resin to which an electroconductive agent such as carbon black is added. When this disclosed technology is applied to such an intermediate transfer belt, since a filler represented by black carbon, etc., is mixed in the polyimide film as an electroconductive agent and imparts semi-conductive properties. Thus charged toner particles can be transferred from a transfer drum to an intermediate transfer belt because of this filler. This is true for the case of transfer to a printing paper.